CO₂ corrosion morphology of X70 pipeline steel under jet impingement at high temperature and high pressure environment

The CO₂ corrosion behavior of X70 pipeline steel, including the corrosion product's morphology, three-dimensional surface topography, and corrosion thickness reduction as well as its statistical analysis, was investigated in high temperature and high pres-sure CO₂ environment using self-developed loop jet impingement apparatus and computation fluid dynamic (CFD) technique. The relationship between the obtained results and flow regimes under jet impingement was also discussed. It is found that the differences of fluid mass transfer and wall shear stress distributed on the steel surface located at different flow regimes are the main reason for the differences of the corrosion product's morphology, three-dimensional surface topography and corrosion thickness reduction. According to the order of the laminar zone, the wall jet zone and the transition zone, the corrosion product is thinned, degraded and even removed from the steel surface because of the continuous increasing of wall shear stress, which will decrease the mass transfer resistance, accelerate the mass transfer rate, and continuously enhance the corrosion process of the steel. Therefore, in the order of the laminar zone, the wall jet zone and the transition zone, the structure of the corrosion product changes from complete and compact to loose and porous, the three-dimensional morphology of the substrate surface changes from flat to steep, the average roughness and the root mean square as well as the average corrosion thickness reduction and the standard deviation gradually increase. The corrosion rate and the wall shear stress can be correlated properly with the exponential relationship.